Modern disk files usually comprise one or more magnetic data storage disks mounted for rotation about a common axis. Data is stored on the disks in concentric tracks, and is read from or written to the disk surfaces by transducing heads. There is generally one read/write, or data, head per disk surface, although only a single head/disk surface combination is actually used to read or write user data at any given time. Typically a write signal is supplied to a multiplexer, which then passes the signal onto whichever head is currently selected for writing. Likewise, a read signal is obtained from a chosen head via a separate, read multiplexer. All the heads are ganged together for common motion on an actuator, which moves the heads radially in and out across their respective disk surfaces. Movement of the actuator itself is controlled by a servo system, utilizing servo information recorded on one or more of the disks. By reading this servo information, the actual positions of the heads can be determined, and after comparison with the desired head positions, control signals can be sent to move the actuator accordingly.
Heads for disk files conventionally comprise inductive read/write coils, but another known form of head uses the magnetoresistive (MR) effect to read data from the disk surface, this generally providing a stronger read signal, especially for low velocity media. Since the MR effect cannot be used for writing data, an MR head typically comprises an MR stripe made of a material such as Permalloy for reading data, located between the pole tips of a conventional inductive write coil (thin film or otherwise). Examples of MR heads are described in EPB 0021392 and EPB 0100841.
Two types of servo system are commonly used to control the head positions. In the first, a dedicated servo system, one whole disk surface (a servo disk) is used exclusively to store servo information on. A servo head constantly reads this information to provide a continuous signal indicating the position of the servo head with respect to the servo disk, and by extension, the location of all the other data heads on that actuator. The use of a dedicated servo however has two main disadvantages. Firstly, offsets can occur between the servo head and the data heads, for example due to actuator or disk spindle tilt caused by thermal expansion effects. Secondly, there is a reduction in the data storage capacity of the device because a whole disk surface which could potentially store user data instead contains only servo information. EPA 0267771 discloses a disk drive with one servo disk at the top of the disk stack and one at the bottom, wherein problems of spindle tilt are compensated for by linear interpolation between the two servo heads. This improves the track following performance, but at the cost of doubling the servo overhead.
In the second type of servo system, sectors of servo information are interspersed with sectors of data on each disk surface (this type of system is known as a sector servo). As a head follows the data track around, it regularly reads a fresh sample of servo information from each servo sector to control its position with. A sector servo system therefore has no problem with offsets, because the servo signal is obtained from whichever head is actually reading or writing data. Potentially at least, this allows more accurate track following than with a dedicated servo system, in turn permitting higher storage densities.
The servo overhead in a sector servo system, which is distributed between all the disks, may be quite large. This is firstly because it is difficult to greatly reduce the size of individual servo sectors, since each must contain the necessary positional information, preceded by a guard region to protect against accidental overwriting. Secondly, in order to improve track following and to reduce access times, it is desirable to have a high sampling frequency of servo information, which means as many servo sectors as possible. However, increasing the number of servo sectors reduces the amount of disk surface available to store user data on. Thus a sector servo system suffers from an undesirable trade-off between high storage capacity and low access times.
GB 2191877 discloses magnetic disk apparatus that instead of just utilizing servo sectors on the disk surface that is being read from, multiplexes sector servo samples from all the disk surfaces. The servo sector patterns are offset in phase from one disk surface to another, so that as the disks rotate, a servo sector from one disk surface is read, then from another, and so on, until a servo sector from each disk surface has been read, whereupon the sequence repeats. The same basic idea is also disclosed in "Disk file controlled by Multiplexed Servo Samples", I Jones, IBM Technical Disclosure Bulletin 28/8 p. 300, January 1986. Multiplexing servo samples from different disk surfaces produces a higher servo sampling rate without any increase in the number of servo sectors per disk--i.e., without increasing the servo overhead.
The apparatus disclosed in GB 2191877 is read-only, whereas usually the storage capacity of disk files is determined by how accurately the heads can follow the desired track when writing, thereby avoiding any accidental erasure of neighboring tracks. The prior art does not suggest a disk file which is capable of providing a multiplexed sector servo signal during data write operations.